1. Field of the Invention
The present invention relates to pneumatic vibration isolators.
2. Background Information
Vibration isolators are used to isolate vibration and shock loads applied to a working surface. For example, vibration isolators typically support the table of semiconductor fabrication equipment to isolate shock and vibration that would otherwise be caused by transmission of vibration from the floor to the table through the table support.
Some vibration isolators contain a piston that moves within an air filled cylinder. For example, U.S. Pat. No. 3,784,146 issued to Matthews and U.S. Pat. No. 5,071,108 issued to Houghton disclose vibration isolators that contain a pneumatic piston which is coupled to a support plate by a plurality of cables. The support plate is typically attached to a payload such as a table. The support plate may have a shaft that extends down into a damping fluid. Horizontal movement of the payload and attached support plate may be damped by movement of the shaft within the damping fluid. The cables allow the support shaft to translate within the piston.
The vibration isolator may be coupled to an air leveling valve that can vary the air pressure within the isolator and compensate for large system disturbances. By way of example, the disturbance may be caused by removing or adding weight to the payload, which causes an immediate upward or downward movement of the support plate, respectively. The isolators are typically designed to provide a relatively high leveling gain and vertical damping to quickly compensate for the system disturbance. Unfortunately, the high leveling gain may cause system oscillation. The high vertical damping may degrade isolation efficiency.
One embodiment of the present invention includes a vibration isolator with a flange that extends from a piston. The piston is coupled to a housing by a diaphragm. The diaphragm and flange define a balancing chamber that is in fluid communication with a leveling port. The flange is separated from the housing by a gap that provides fluid communication between the balance chamber and an inner chamber within which the piston moves relative to the housing.